1. Field of the Invention
The present invention relates generally to brushless direct current (DC) motors and more particularly to a limited angle torque DC brushless motor for use in pneumatic servovalves and a variety of other applications.
2. Prior Art
Pneumatic servovalves are employed wherever there is a need for high-precision, closed loop motion control systems such as in automotive vehicle component testing and manufacturing, semiconductor manufacturing, packaging, equipment assembly, industrial automation and robotics, lumber processing and the like. Limited angle torque motors for use in pneumatic spool and sleeve servovalves usually comprise a DC brushless motor, a Hall sensor and an integrated controller. The limited angle torque motor drives the valve spool through an eccentric which is built into the motor shaft. Rotary operation of the motor shaft is translated into linear motion of the spool which modulates the gas flow through the control ports of the servovalve. The spool position is monitored by the Hall sensor with the controller comparing spool position with the valve input command. The resulting difference generates a current signal from the controller which drives the valve spool to the desired position.
Conventional DC brushless motors are widely used in limited angle torque applications and have satisfactory and reliable performance but are relatively expensive components for the servovalve manufacturer which is a major disadvantage as a significant portion of the retail cost of a pneumatic servovalve is directly related to the cost of the integrated DC brushless motor. DC brush motors, on the other hand, are relatively inexpensive but are never used in such applications as DC brush motors do not possess the high cycling and high frequency capability demanded by servomotors of this type. The problem lies in the commutator/brush interface with the brush/commutator connection becoming unreliable and impractical to use with high cycle and high frequency use. Carbon brushes typically tend to wear out quickly in high cycling and/or high frequency applications due to friction between the brushes and the commutator segments. Furthermore, brush control in high cycle limited angle torque applications is a major problem whereby complex and expensive controller circuitry may be needed to overcome the problem which, however, would further raise the cost of the servomotor and is undesirable. Therefore, the need arises for a low cost limited angle torque DC brushless servomotor which can provide reliable performance thereby presenting a viable alternative cost efficient solution for servovalve customers. Such a motor should preferably be easy to assemble in very short time and should be constructed from relatively inexpensive yet reliable components.